The present invention relates to the art of laser welding.
Japanese Patent Application Laid-Open No. 328,277/1994 discloses a laser welding method in which a YAG laser torch is caused to travel at a given rate along members which are to be welded together while feeding YAG laser pulses to the torch through an optical fiber to form one welded spot per pulse, thus forming a train of welded spots at a given pitch along the members to be welded.
Japanese Patent Application Laid-Open No. 314,973/1998 discloses a laser machining apparatus which uses a composite laser beam to perform a high power welding at a high rate. At this end, YAG laser pulse and a continuous laser wave of a different variety are passed through a single or a plurality of optical fibers to feed a condenser lens of a machining head, which irradiates members to be welded.
Japanese Patent Application Laid-Open No. 351,086/2000 discloses a laser beam welding method in which laser pulses are projected for welding purpose at a pulse interval such that before a key-hole which is created by the projection of a preceding laser beam pulse to cause a local melting and evaporation of a member or members is closed, a succeeding laser beam pulse is projected to the key-hole.
In summary, YAG laser welding has features as mentioned below and is extensively used in the automatic welding process for automobiles.
1) A focused laser radiation is obtained and enables a welding operation with a low distortion and at a high rate;
2) Laser radiation absorption coefficient for metal materials is several times higher than for a CO2 laser, thus allowing an efficient welding operation. Since the wavelength of the laser radiation is on the order of {fraction (1/10)} that of CO2 laser, attenuation caused by plasmas generated during the welding operation is reduced.
3) The laser radiation can be transmitted through a flexible optical fiber, and thus a handling is facilitated and the laser can be used with a multi-joint robot. The transmission to a location which is located as far as 100 meters is possible.
4) The laser radiation can be used in a time division manner or spatially split (or power divided) manner, and this allows the distribution of the laser radiation to a plurality of machining stations to achieve a high utilization efficiency.
On the other hand, the welding operation by using YAG laser alone involves the following issues:
1) YAG laser is used in the welding operation by condensing the beam radiation to achieve an increased power density. However, the condensed spot has a very small diameter on the order of xcfx860.3 to xcfx860.1 mm, and this causes the following inconveniences;
For butt welding, the presence of a gap between members to be welded which are disposed in abutment against each other (see FIG. 3a) allows the laser radiation to pass through the gap, causing weld defects. This imposes a severe demand on the cutting of an end face of a member to be welded where a weld is to be formed as well as on the manner of constraining parts of the members located adjacent to the weld.
For lap welding, the width of faying interface of a bead which extends from a top member (see FIG. 3b) to a bottom member is limited, preventing a satisfactory bonding strength from being obtained.
For fillet welding, a satisfactory depth of fusion (see FIG. 3c) cannot be obtained in the similar manner as mentioned above, preventing a satisfactory bonding strength from being obtained.
2) YAG laser apparatus requires a high equipment cost or initial cost, and therefore, where a plurality of equipment must be provided as in an automobile production line, a prohibitive installation cost results.
It is known to divide a single laser beam in an optical system which guides a laser radiation to a machining head as by the use of a prism so that a plurality of spots may be formed. For example, a pair of spots (or twin spots) may be defined across a weld line in order to avoid the occurrence of a pass-by as would occur in a single spot arrangement. This provides a technique which would improve the margin demanded for a butt welding gap. However, the division of the laser beam into a plurality of sub-beams results in a power for each spot which is equal to the laser output p divided by the number N of the spots. This means a reduction in the power delivered to each spot, and there arises a problem that there must be provided a laser having a higher output capability.
It is an object of the present invention to overcome problems mentioned above by enabling a defect-free laser welding in a butt welding operation without demanding a higher precision in the butt welding gap maintained, achieving a greater width of facing interface and a greater depth of fusion in the lap welding operation and the fillet welding operation, respectively, by a laser welding and obtaining a higher joint strength in either instance.
The present invention overcomes the described problems by the use of a pulse laser output to twin spots. According to the twin spots pulse laser welding method,
(1) there can be achieved a deep depth of fusion with a peak output even if a laser having a mean output which is equivalent to a continuous wave laser is used;
(2) For example, when welding together aluminium members, an irradiation laser power equal to or greater than 2 KW is required for a xcfx860.6 mm spot because of the surface oxide film and the reflectivity. However, when pulses are used, a welded operation is made possible using a laser having a smaller output. By way of example, when a laser having a 3 KW is used, the output distributed to twin spots result in 1.5 KW per spot, which prevents a welding operation from being performed. However, if the laser is operated such that it provides 4 KW output during one-half period and to provide 2 KW output during the remaining half period, the mean output of the time sequence will be equal to 3 KW, allowing 2 KW to be supplied to one spot for an interval which is equal to one-half period. This permits a deep depth of fusion to be produced, thus increasing the joint strength;
(3) For lap welding operation, a width of faying interface which is equivalent to the sheet thickness will be normally required in view of the required welding strength, but such width cannot be maintained with the single spot. On the other hand, according to the continuous twin spots welding operation, even though the width of molten top can be increased, the depth of fusion is reduced and the welds in the faying interface are separate from each other, also making it difficult to maintain the welding strength. However, when the pulse output is combined with the twin spots welding, the fusion increases in depth periodically, and in the deep portion, the molten pools produced by the adjacent two spots overlap each other to increase the width of the faying interface, allowing the desired welding strength to be secured.
Above and other objects and features of the present invention will become apparent from the following description of an embodiment thereof with reference to the drawings.